合并两个有序链表（Java实现）

示例：
输入：1->2->4, 1->3->4
输出：1->1->2->3->4->4

package Day47;
/**
 * @Author Zhongger
 * @Description 将两个有序链表合并为一个新的有序链表并返回。新链表是通过拼接给定的两个链表的所有节点组成的。
 * @Date 2020.3.20
 */
public class MergeTwoListsSolution {
    public static void main(String[] args) {
        ListNode l1_1 = new ListNode(1);
        ListNode l1_2 = new ListNode(2);
        ListNode l1_3 = new ListNode(4);
        l1_1.next=l1_2;
        l1_2.next=l1_3;
        ListNode l2_1 = new ListNode(1);
        ListNode l2_2 = new ListNode(3);
        ListNode l2_3 = new ListNode(4);
        l2_1.next=l2_2;
        l2_2.next=l2_3;
        MergeTwoListsSolution solution = new MergeTwoListsSolution();
        ListNode mergeTwoLists = solution.mergeTwoLists(l1_1, l2_1);
        ListNode temp=mergeTwoLists;
        while (temp!=null){
            System.out.println(temp);
            temp=temp.next;
        }
    }
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if (l1==null&&l2==null){
            return null;
        }
        ListNode resList=new ListNode(0);//头结点
        ListNode temp=resList;//辅助节点
        while (l1!=null&&l2!=null){
            if (l1.val<l2.val){
                temp.next=new ListNode(l1.val);
                l1=l1.next;
            }else {
                temp.next=new ListNode(l2.val);
                l2=l2.next;
            }
            temp=temp.next;
        }
        if (l1==null){
            temp.next=l2;
        }
        if (l2==null){
            temp.next=l1;
        }
        return resList.next;
    }
}
class ListNode {
    int val;
    ListNode next;
    public ListNode(int val) {
        this.val = val;
    }
    @Override
    public String toString() {
        return  val+" ";
    }
}

 /**
     * 递归求解
     * @param l1
     * @param l2
     * @return
     */
    public ListNode mergeTwoLists2(ListNode l1, ListNode l2) {
        if (l1 == null) {
            return l2;
        }
        else if (l2 == null) {
            return l1;
        }
        else if (l1.val < l2.val) {
            l1.next = mergeTwoLists(l1.next, l2);
            return l1;
        }
        else {
            l2.next = mergeTwoLists(l1, l2.next);
            return l2;
        }
    }